1. Field of the Invention
The invention relates to the field of measuring apparatus. More particularly, the invention relates to the apparatus for measuring and indicating the tension on a continuous web in a web-winding process.
2. Description of the Prior Art
In a typical web-handling process, a continous web of material is guided over and under a series of tension rollers at high speed as the web is unwound or wound into a finished product. The material may be any thin, flexible material, such as paper, film, metal, etc. Maintaining proper tension on the web is critical in obtaining an evenly and tightly wound roll and/or product. Load cells are typically mounted at the ends of the tension rollers to measure the tension on the web as it moves past the tension roller and generate signals proportional to the magnitude of the tension. Control apparatus continuously adjust the web-winding machinery as a function of the output signals, to ensure that a closely toleranced tension is exerted on the continuous web.
One type of load cell used in the typical web-winding process is an “under-pillow-block” (UPB) load cell that is mounted beneath the pillow block bearing of the tension roller and which is hereinafter referred to as a UPB load cell. The conventional UPB load cell comprises a body with a removable pillow block plate, a load beam, a hinge, and a bottom plate that has mounting flanges at two ends for mounting the UPB load cell to a machine support, such as a stanchion or pillar. The load beam and hinge are assembled between the body and the bottom plate. The load beam assembly includes a cantilevered load beam that is fixedly mounted at one end to a mounting block and to the machine support. The free end of the beam is assembled between two roller pins. Depending on the load applied to the load cell, the body deflects the load beam and the hinge. The free end of the load beam bears the load and deflects as the body moves toward it, while the hinge pivots accordingly about a pivot point.
Conventional strain gauges are applied to the load beam in a bridge configuration in order to obtain output signals that are proportional to the tension on the web. The output signals are used to regulate operation of devices that control the torque of the web-winding process, such as pneumatic brakes or torque regulators on a winding shaft.
The conventional UPB load cell has several disadvantages. Mounting the load beam is a time-consuming task that requires extremely precise assembly of the various mounting components and shims, with sophisticated measuring and adjustment steps. Thus, repair or maintenance work on the load beam assembly generally requires a person of special skills and special tools. Consequently, the load beam assembly or the sensors, which constitute only a fraction of the overall weight of the load cell, but which are typically the part of the load cell that require maintenance or repair, generally cannot be serviced or repaired in the field. If the load beam or the sensors require maintenance or repair, the entire load cell is removed from the production line and sent back to the load cell manufacturer or designated facility. In order to hold costly downtime to a minimum, the production facility is obliged to keep spare load cells in stock.
The conventional UPB load cell, with its removable top mounting plate, mounting block, and bottom plate, has unnecessary bulk and weight, which add to the expense of manufacturing, shipping and handling the load cell. In addition to these disadvantages, the combination of the three separate plates results in a height dimension that significantly influences the bending moment that a load exerts on the load cell and that must be compensated for when mounting the load cell in the production line.
UPB load cells are frequently used in wet environments and, for this reason, a high ingress protection (IP) rating is desirable. With the conventional UPB load cell, the load beam and hinge are fastened to the top mounting plate, from the top of the plate, with bores extending from the top of the load cell down through the body and through the load beam. This provides avenues of ingress into the area of the load beam that carries sensors and electrical connections. A UPB load cell construction that eliminates or significantly reduces the avenues of ingress and therefore improve the IP rating over that of the conventional UPB load cell is desirable.
What is needed, therefore, is a UPB load cell that provides ready access to the load beam and sensors, simplifies assembly and adjustment, and enables in-field replacement of the load beam and/or sensors. What is further needed is such a UPB load cell that provides greater ingress protection. What is yet further needed is such a UPB load cell that is lighter in weight and more cost-efficient to manufacture.